Why do you tip based on a % of your bill? What if you came with some friends and you all snacked on appetizers and tossed back some drinks, requiring more frequent stops to your table but netting a relatively small bill. Do you still tip 15-20%?

What if an Applebee's server gave you great service but a server at a more high end restaurant gave you just above average service, do you tip the above average guy more because your bill was more?

I just don't get why people gauge their tips based on what their meal cost, and not on what they feel the server actually deserved for what they did. Or, for that matter, why 2-3 bucks is considered a bad or average tip.